1. Field
The present invention relates to a method of designing a projection system, a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The present invention is concerned with designing the projection system (also referred to as an optical system) for imaging the pattern onto the substrate. There are many constraints that must be met when devising a projection system and many variable parameters. The process becomes an optimization problem. The impressive progress in the global optimization of optical systems over the past two decades has resulted in powerful software tools. For optical designs for which the complexity is not too high, present-day global optimization algorithms are valuable tools for finding a good solution among the many local minima that are found in the merit function landscape. However, if the number of components is large, or if there are a large number of variable parameters such as aspheric coefficients, then even local optimization is time consuming, and it is difficult to apply such tools straightforwardly. In this case, local minima that differ from the known ones must be found with methods that use only a small number of local optimizations to achieve their goal. A further problem is that, even starting from known designs, it is difficult to generate new projection systems in a simple, efficient and systematic manner.
Therefore, what is needed is a method for designing new projection systems for lithographic projection apparatus